A couple of things......
This isn't a huge project. Even so you'll need some Colloidal Silica or better yet Cabosil to thicken your resin. You'll also need some Q-cells (these are lightweight micro balloons that sand easily) to make up a fairing compound. Read that Epoxy Book and you'll understand this.
#4 If you can use a 1/2 inch round over bit so much the better.
#5 No need to tape outside corners. Just cover the box with additional layers to build up the laminate.
#7 Counter sink any screws that you use (304 stainless minimum, 316 preferred) then just fill the counter sink with epoxy and sand smooth. Just lay up your cloth right over the countersunk screws.
#9 You can make fillets more easily using the "rubbing alcohol trick". Thickened resin has a tendency to stick to everything. It's difficult to get a smooth fillet. Once you've smoothed your fillet as best you can allow the resin some time to reach a plastic or semi-solid state. This state will last for a longer period of time with medium to even slow hardeners. While the resin is in this state wet a latex gloved finger with isopropyl rubbing alcohol. The alcohol will allow you to smooth the fillet and work it into corners and shape it without the glove sticking. You can also use wooden cleats on inside corners. Depending on the angles involved this can be a good option.
#13 I've never waited a week before painting. Just follow the manufacturers guidance regarding cure times.
#14 I sand with 120 grit unless I have a really tricky fillet then I get more aggressive (maybe 60 or 40 grit) and then recoat the fillet until I get it right.
#15-16 Just follow the directions with the epoxy prime coat and the finish paint. If you use Perfection follow the instructions TO THE LETTER. I'd practice on a scrap piece, maybe make up a scrap piece coated with resin while you're making the box. I find that 3" rollers and disposable chip brushes work fine. You want a thin nap roller. Also keep in mind that with Epoxy Prime coat you'll need brushing/rolling thinner at 25%. You'll understand why when you open the can. For Perfection you'll thin more when it's cool and less when it's in the 80's or 90* but never more than the max that the manufacturer recommends, 10% I think it is. When you first try using Perfection, thinning can be tricky. If you think you want 10% try just a splash first and see how it rolls out. You can then add a bit more if necessary and "sneak up" to the best mix. If it's hot try Perfection with no thinner at all, you'll get a nice result. Pot life is pretty long with Perfection so you'll have time to play around with it. One more thing, with Perfection "less is more". It's a thin product that covers very well. Lay on 2-4 thin coats. If you try to go heavy it will run on you. Thin, thin , thin coats.
These are the rollers I use. You can cut them with a hack saw and miter box to fit a three inch roller frame.
https://www.amazon.com/Wooster-Brus...locphy=9004606&hvtargid=pla-511352533392&th=1
Good Luck,
MIA